Annealing process for magnetic steel strip



ANNEALING PROCESS FOR MAGNETIC STEEL STRIP Elmore J. Fitz, Lanesboro,Mass, and Robert E. Waite, East 'Chatham, N.Y., assignors to GeneralElectric Company, a corporation of New York No Drawing. Filed July 28,1958, Ser. No. 751,128

8 Claims. (Cl. 148112) The present invention relates to magnetic siliconsteel for electrical uses such as in transformers, motors and otherelectromagnetic apparatus, and more particularly to a process ofannealing silicon steel which is adapted to be carried out on acontinuous basis.

In the processing of silicon steel strip to produce material of goodmagnetic and electrical properties, the strip is normally subjectedafter a series of rolling stages to an annealing treatment in which twopurposes are sought to be accomplished. First, the anneal should developin the steel a crystal structure (texture) so oriented that goodmagnetic properties are obtained in the strip. Such crystal structure isdeveloped by secondary recrystallization, as explained below. Second,the anneal should remove impurities such as sulfur and carbon which maycause excessive watt loss in the oriented strip. In general, low wattloss cannot be obtained without good crystal orientation and withoutsuitable purification of the material.

By secondary grain growth or secondary recrystallization as referred toherein is meant the process whereby in the final texture producingannealing treatment, strainfree crystal grains grow in size by absorbingeach other. Such secondary grain growth follows primaryrecrystallization, which is a process whereby the distorted grainstructure of a cold-worked metal is replaced by a new strain-free grainstructure by annealing above a specific minimum temperature. It is thesecondary recrystallization that produces the highly preferredorientation sought in high quality magnetic strip, and the orientationthus obtained is completely different from that obtained merely afterprimary recrystallization. The magnetic strip is generally in a primaryrecrystallized state when it is ready for the final high temperatureanneal. The grains in a primary recrystallized material are on theaverage considerably smaller than those of a secondary recrystallizedmaterial and, also, while primary recrystallized strip has only a smallpercentage of orientation, e.g. to the large grained secondaryrecrystallized strip has a much higher degree of orientation with valuesof 70 to 95% being the rule.

It has previously been found that in order to develop the maximum degreeof secondary recrystallization in the annealing stage, the heattreatment is best carried out at a temperature which is lower than theoptimum purification temperature. For example, a temperature of 850 C.for grain growth and of 1100 C. for purification has been taught by theprior art. In such prior processes the steel strip was held at the lowertemperature for a substantial period of time, at least several hours, toallow adequate grain growth (secondary recrystallization), after whichthe strip was subjected for several more hours to the higher purifyinganneal temperature.

These and other relatively long annealing treatments heretoforeconsidered necessary have been a major obstacle to the development of acontinuous annealing treatment in commercial processing of silicon steelstrip. Due to the necessity for prolonged treatment in the annealingfurnace, the steel strip has conventionally been proc- Patented May 30,1961 essed in batches in the furnaces, either in the form of coils orstacked laminations. This procedure has many drawbacks. For one thing,it resulted in lack of uniformity in electrical and magnetic propertiesbetween different batches or coils, or even between different portionsof the same strip. Also it necessitated the application of separatorcoatings between layers of the strip to prevent them from stickingtogether durin the annealing process. In the case of the coiled strips,these had to be subsequently flattened by an additional heat treatmentto overcome the coil set. A continuous anneal, on the other hand,wherein a single strand of steel is continuously passed through theannealing furnace would not be subject to such difliculties and wouldmarkedly facilitate commercial processing of electricalsteel.

It is an object of the present invention to provide an improved methodof processing silicon steel which overcomes the above disadvantages ofprior processes.

It is another object of the invention to provide a method of treatingsilicon steel which may be carried out in a continuous process.

It is a particular object of the invention to shorten the time requiredfor the final texture-producing and purifying anneal treatment in theprocessing of electrical steel so as to adapt such treatment to acontinuous process, while affording steel of a high degree of magneticanisotropy and very low watt loss and other characteristics of highquality electrical steel.

Other objects and advantages of the invention will become apparent fromthe following description and the appended claims.

To attain the above objects, the present invention provides a method oftreating silicon steel strip containing about 1 to 4% silicon whichcomprises the steps of heating the silicon steel material for about 5 to60 minutes, and preferably about 15 minutes, at a temperature of about950 to 1050 for the purpose of initiating the nucleation and growth ofcrystals of preferred magnetic orientation, and thereafter heating thematerial at a temperature of about 1175 to 1250 C. to provide forcontinued growth of crystals of the preferred orientation. For bestresults, the cooling rate of the thus annealed material is carefullycontrolled as described hereinafter.

It has been unexpectedly found in accordance with the invention that thelow temperature period for initiating secondary recrystallization may bethus considerably reduced below that heretofore considered necessary andthat completion of secondary recrystallization can be achieved duringthe subsequent higher temperature purifying stage to produce highquality steel strip.

A series of tests were made on samples of silicon steel strip containingabout 3%% by weight of silicon to determine the effect of dilferenttemperatures and periods of heating on the development of magneticanisotropy in the samples.

The degree of anisotropy produced in the tested samples is shown inTable I below as percent orientation, these values being obtained by theintegrated disc test well known in the art. The latter test involves theuse of an instrument which compares the voltage developed in a magneticflux field by a disc of the test sample with that developed-with astandard specimen having a known degree of magnetic anisotropy. Sincethe degree of magnetic anisotropy is a function of the amount oftexture, i.e., crystal orientation in the strip, it is possible tocallbrate the disc tester in terms of percent of perfect crystalorientation.

In the annealing tests, the specimens were initially heated in a tubefurnace at selected nucleating temperatures of 950 C., 975 C., 1000 C.and 1025 C. and thereafter given a heat treatment of 1175 C. for 8hours. Table I below shows the results of these tests at different timesand temperatures, the values shown being in percent anisotropy based onan average of samples:

Table l Percent Nucleating'lemp. and Time Anisot- 30 min 93 60 min 93975 0.:

10 min 93 30 min 91 60 min 89 1,000 0.:

8 min 90 m 88 1,026" 0.:

15min 84 As a basis for comparison, 30 samples of similar silicon steelmaterial were given a standard anneal at 1175 C. for 8 hours without anyprior heat treatment. Such anneals are batch type and the thermalinertia of the stacked pieces insures that they will pass through thegrain growth temperature range slowly enough to allow sufficient timefor proper texture development in accordance with the teaching of theprior art. The average value for these 30 samples was 86% anisotropy.

it will be seen from these results that in all cases where the initialheat treatments were in the range of 950 C. to 1000 C. the anisotropyobtained was greater than that obtained in the samples given thestandard 1175 C. anneal. It is also noteworthy that the shorter times ateach temperature resulted in as high or higher a degree of magneticanisotropy as the longer periods at the same temperature. The resultsobtained were further unexpected in demonstrating that substantiallyhigher secondary recrystallization temperatures than heretoforeconsidered desirable could provide entirely satisfactory degrees oforientation.

In connection with the above tests, samples were checked for percentageorientation at the end of various times at the respective nucleatingtemperatures. From this it was found that the time for completeanisotropic development at these temperatures was between 1 and 16hours. It seemed evident, therefore, that in the procedure carried outas described only a small degree of secondary recrystallization isdeveloped during the relatively short periods at the lower temperaturesand that the necessary grain growth was for the most part accomplishedand completed during the higher purifying anneal stage. This is incontrast to the prior art teaching which has assumed thatadequatesecondaryrecrystallization could be carried out only byholdingthe steel strip at temperatures below 900 C. for periods of not lessthan four hours duration.

It has further been found in connection with the invention that if thesecond stage anneal is carried out at a temperature in the vicinity of1200 C. and the rate of cooling thereafter is carefully controlled to amaximum of 100 C. per minute a substantial reduction can be made in thetime required for the second stage anneal with little or no sacrifice inthe quality of steel produced.

Table 11 below shows the results of tests conducted on silicon steelstrip using the two stage treatment of the invention at l000 and then1200 C., the percent texture shown being the average value of the twelvelots From these results it is again definite that good crystaldevelopment can be achieved by a partial growth for a short time at alow temperature followed by completion at a higher temperature. The dataalso indicates that lengthening the period of the first stage permits ashortening of a second stage to attain similar or better, percenttexture.

On the basis of numerous tests, a particular annealing cycle wasestablished which proved suitable for consistently producing highquality magnetic steel. This cycle consisted of heating the siliconsteel strip for about 15 minutes at 1000 C., 15 minutes at 1225 C., andcooling at a maximum rate of C. per minute. This cycle calls for a totalelapsed time of about 42 minutes, a period which is considerably shorterthan the many hours of annealing treatment customary in the prior artprocesses. The watt losses of ten lots of silicon steel samples 14 milsthick at a flux density of 15,000 subjected to the foregoing cycle werefound on the average to be about'.64 watt per pound, and the magneticanisotropy was 92%, values which are quite satisfactory, especiallyconsidering the rapidity of the annealing process.

As a consequence of the markedly shortened annealing periods thusprovided, it becomes practical to treat the steel strip in a continuousannealing process. Thus, a continuous anneal may be carried out bymoving cold rolled silicon steel strip of indefinite length andtypically 14 mils thick through adjoining furnace chambers having thetemperature levels set forth in the foregoing cycle, i.e., 1000 C. and1225 C., and at a speed such that it takes 15 minutes for movementthrough each furnace chamber. Thereafterthe strand may be moved througha cooling chamber where it is cooled at the stated maximum rate of 100C. per minute.

While the above described cycle appears particularly satisfactory forthe purposes of the invention, the invention is not limited to thespecific times and temperatures stated therefor. Tests have shown, forexample, that the 15 minute period for the second stage anneal at 1225C. could be further reduced without substantial sacrifice in watt lossor degree of crystal orientation. Material so treated at 1225 C. in thesecond stage for 10 minutes showed a watt loss of .65 watt per pound anda percent texture of 92%, while that similarly treated for 5 minutes hadthe same watt loss and a percent texture of 91%, these values each beingbased on an average of five samples. In general, the initial stagetemperature may be in the range of 900 C. to 1050 C. and for a period of5 to 60 minutes, and the secondstage temperature in the range of 1175 C.to 1250 C. for a period of l to 30 minutes.

It has further been found that in order to obtain the benefits of theinvention it is essential that'the second stage high temperature annealfollow immediately upon the first stage low temperature treatmentwithout any intervening cooling period, since tests have indicated thatwhere an intermediate cooling period is provided, the percentorientation ultimately achieved-was substantially lower than in theabsence of such an intervening cooling period.

The'annealing treatments described are preferably carried out inreducing atmospheres in the furnace, and while hydrogen may typically beemployed for this purpose, other known types of reducing atmospherescould be used if desired. a

The expressions sheet material and sheet as used in the appended claimsare intended to include such forms as sheets, strips, tapes and otherlaminar shapes.

While the present invention has been described with reference toparticular embodiments thereof, it will be understood that numerousmodifications may be made by those skilled in the artwithoutactual-lydeparting from the scope of the invention. Therefore, the appendedclaims are intended to cover all such equivalent variations ascomewithin the'true spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. The method of treating cold rolled electrical silicon steel stripcontaining 1 to 4 percent silicon which comprises the steps of heatingsaid strip for 5 to 60 minutes at a temperature of 950 to 1050 C. toinitiate secondary recrystallization therein, and immediately thereafterheating said strip at a temperature of 1175 to 1250 C. without coolingthe same between the heating steps for providing continued secondaryrecrystallization in said silicon steel strip.

2. The method of treating cold rolled electrical silicon steel stripcontaining 1 to 4 percent silicon which comprises heating said siliconsteel strip for to 30 minutes at a temperature of 950 to 1050 C.,immediately thereafter heating said strip for l to 30 minutes at atemperature of 1200 to 1250 C. without cooling the same between theheating steps, and thereafter cooling said strip at a rate of not morethan 100 C. per minute.

3. The method of treating cold rolled electrical silicon steel stripcontaining 1 to 4 percent silicon which comprises heating said siliconsteel strip for about minutes at about 1000 C., immediately thereafterheating said strip for about 15 minutes at about 1225 C. without coolingthe same between the heating steps, and thereafter cooling said strip ata rate of not more than 100 C. per minute.

4. The method of processing of electrical silicon steel sheet materialcontaining 1 to 4% silicon which comprises continuously moving anelongated strip of said material in sequence through a reducingatmosphere at a temperature of 950 to 1050" C., and then immediatelythrough a reducing atmosphere at a temperature of 1175 to 1250 C.without cooling the material between the heating steps.

5. The method of processing electrical silicon steel sheet materialcontaining 1 to 4 percent silicon which comprises continuously moving anelongated strip of said material through a reducing atmosphere at atemperature of 950 to 1050 C. and then immediately through a reducingatmosphere at a temperature of 1200 to 1250 C. without cooling thematerial between the heating steps, the speed of said material beingsuch as to subject the same to aforementioned temperatures forrespective periods of about 10 to 30 minutes and 1 to 30 minutes.

6. The method of processing electrical silicon steel sheet materialcontaining 1 to 4 percent silicon which comprises continuously moving anelongated strip of said material through a reducing atmosphere at atemperature of 950 to 1050 C. and then immediately through a reducingatmosphere at a temperature of 1200 to 1250" C. without cooling thematerial between the heating steps, the speed of said material beingsuch as to subject the same to the aforementioned temperatures forrespective periods of about 10 to 30 minutes and l to 30 minutes, andthereafter cooling said strip at a rate of not more than 100 C. perminute.

7. The method of processing electrical silicon steel sheet materialcontaining 1 to 4 percent silicon which comprises continuously moving anelongated strip of said material through a reducing atmosphere at atemperature of about 1000 C. and then immediately through a reducingatmosphere at a temperature of about 1225 C. without cooling thematerial between the heating steps, the speed of said material beingsuch as to subject the same to the aforementioned temperatures forrespective periods of about 15 minutes each, and thereafter cooling saidstrip at a rate of not more than 100 C. per minute.

8. In the processing of electrical silicon steel sheet materialcontaining 1 to 4% silicon, the steps of heating such sheet material for5 to minutes at a temperature of 950 to 1050 C., and immediatelythereafter heating said sheet material at a temperature above about 1175C. without cooling the same between said heating steps.

References Cited in the file of this patent UNITED STATES PATENTS1,965,559 Gross July 3, 1934 2,209,687 Crafts July 30, 1940 2,303,343Engel et al Dec. 1, 1942 2,307,391 Cole et a1. Jan. 5, 1943 2,534,141Morrill et a1 Dec. 12, 1950 2,867,559 May Jan. 6, 1959 FOREIGN PATENTS778,987 Great Britain July 17, 1957

2. THE METHOD OF TREATING COLD ROLLED ELECTRICAL SILICON STEEL STRIPCONTAINING 1 TO 4 PERCENT SILICON WHICH COMPRISES HEATING SAID SILICONSTEEL STRIP FOR 10 TO 30 MINUTES AT A TEMPERATURE OF 950 TO 1050*C.,IMMEDIATELY THEREAFTER HEATING SAID STRIP FOR 1 TO 30 MINUTES AT ATEMPERATURE OF 1200 TO 1250*C. WITHOUT COOLING THE SAME BETWEEN THEHEATING STEPS, AND THEREAFTER COOLING SAID STRIP AT A RATE OF NOT MORETHAN 100*C. PER MINUTE.